Direct no decomposition catalyst

ABSTRACT

An improved catalyst system is provided for the direct decomposition removal of NOx from an exhaust gas stream at temperatures between about 350° C. and about 600° C. that employs an (amorphous CuOx)/Co3O4 catalyst. The catalyst has an amorphous CuOx deposit on the surfaces of particles of Co3O4 spinel oxide. The catalyst is configured to reduce NOx to N2 without the presence of a reductant. The (amorphous CuOx)/Co3O4 catalyst is formed by the precipitation of the deposit from solution onto a suspension of Co3O4 spinel oxide particles. The catalyst system can be employed in a catalytic converter for the direct decomposition removal of NOx from an exhaust gas stream flowing at a temperature of less than or equal to about 500° C.

TECHNICAL FIELD

The present disclosure generally relates to catalysts for treatment ofan exhaust gas stream and, more particularly, to a superior copperoxide/cobalt oxide catalyst for removal of nitrogen oxides from a lowtemperature exhaust gas stream as generated by an internal combustionengine, or the like.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it may be described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presenttechnology.

The catalytic removal of NOx from exhaust emissions is desirable forprotection of the environment and compliance with regulations directedto clean air. Catalysts that convert NOx to inert nitrogen gas overother nitrogen-containing compounds are particularly desirable. NOxreduction catalysts that are effective at low temperature may haveadditional utility for vehicles.

The achievement of greater fuel economy from vehicles and automobileengines with effective removal of NO from their exhausts will requirecatalytic NO abatement technologies that are effective under lean-burnconditions. Removal by direct NO decomposition to N₂ and O₂ is anattractive alternative to NO traps and selective catalytic reduction(SCR). Currently, NO traps and SCR processes are highly dependent onreductants such as unburned hydrocarbons and carbon monoxide to mitigateNO_(x). The development of an effective catalyst for direct NO_(x)decomposition would eliminate the need of reducing agents, simplifyingthe NO_(x) removal process, and therefore decreasing the cost to achievegreater fuel efficiency because of the costs for NO_(x).

Few catalysts active for direct NO decomposition are efficient attemperatures lower than about 600° C., which is not practical forstate-of-the-art vehicle exhaust gas streams, which typically are attemperature below about 500° C. Current catalysts known for directNO_(x) decomposition include: Cu-ZSM5, a Cu⁺² ion-exchanged zeolite;alkali metal cobalt oxide lattice, K/Co₃O₄ and Na/Co₃O₄; Ag/Co₃O₄; andCuO. Unfortunately, none of these catalysts display low temperatureactivity and are selective for NO_(x) decomposition to N₂. Recently, acatalyst comprised of copper oxides dispersed on a spinel Co₃O₄ supporthas achieved good catalytic activity at temperatures as low as about300° C. for the direct conversion of nitrogen oxides to nitrogen gas ina cool lean exhaust gas stream. Still, further improvement in catalystactivity and cost effectiveness for the preparation of a direct NOxdecomposition catalyst system remains desirable.

SUMMARY

This section provides a general summary of the disclosure and is not acomprehensive disclosure of its full scope or all of its features.

In various aspects, the present teachings provide an improved catalystsystem for the direct decomposition removal of NO_(x) from a coolexhaust gas stream. In various aspects, the exhaust stream displays atemperature that is between about 350° C. and about 600° C. The catalystsystem includes a spinel Co₃O₄ support. An amorphous CuO_(x) isdeposited on a surface of the Co₃O₄ spinel oxide, providing an(amorphous CuO_(x))/Co₃O₄ catalyst that is configured to catalyze areduction of the NO_(x) to generate N₂ without the presence of areductant.

In other aspects, the present teachings provide a method to prepare theamorphous CuO_(x)/Co₃O₄ catalyst by a deposition-precipitation of theamorphous CuO_(x) on the support. The deposition-precipitation methodpermits the formation of high loadings of amorphous CuO_(x) on the metaloxide support and very active and selective catalysts for the reductionof NO_(x) to N₂.

In other aspects, the present teachings provide a catalytic converterfor the direct decomposition removal of NO_(x) from an exhaust gasstream flowing at a temperature of less than or equal to about 500° C.In various aspects, the catalytic converter includes an inlet configuredto receive the exhaust gas stream into an enclosure, and an outletconfigured to allow the exhaust gas stream to exit the enclosure. Acatalyst system may be contained inside the enclosure. The catalystsystem may include an (amorphous CuO_(x))/Co₃O₄ catalyst that isconfigured to catalyze a reduction of the NO_(x) to generate N₂ withoutthe presence of a reductant.

In still further aspects, the present teachings provide methods for thedirect decomposition removal of NO_(x) from a low temperature exhaustgas stream. In various implementations, the methods may include flowingthe exhaust gas stream through an enclosure with a catalyst system andcontacting the exhaust gas stream to an (amorphous CuO_(x))/Co₃O₄catalyst surface. The methods include catalyzing a reduction of theNO_(x) to N₂ without the presence of a reductant.

Further areas of applicability and various methods of enhancing theabove coupling technology will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present teachings will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 shows composite x-ray diffraction (XRD) patterns for 5 wt. %loadings of CuO_(x) on Co₃O₄ synthesized using different precipitatingagents.

FIG. 2 shows composite x-ray diffraction (XRD) patterns for 3, 5, 7 and10 wt. % loadings of CuO_(x) on Co₃O₄ synthesized using Na₂CO₃ as theprecipitating agent.

FIG. 3 is composite Co2p x-ray photoelectron (XPS) patterns for(amorphous CuO_(x))/Co₃O₄ catalysts prepared from three differentprecipitating agents.

FIG. 4 is composite Cu2p x-ray photoelectron spectrometry (XPS) patternsfor (amorphous CuO_(x))/Co₃O₄ catalysts prepared from three differentprecipitating agents.

FIG. 5 is composite plots of direct NO decomposition activity for(amorphous CuO_(x))/Co₃O₄ catalysts having 5 wt. % loading of CuO_(x)prepared from three different precipitating agents.

FIG. 6 is composite plots of direct NO decomposition activity for(amorphous CuO_(x))/Co₃O₄ catalysts prepared with different CuO_(x)loadings precipitated using from Na₂CO₃.

It should be noted that the figures set forth herein are intended toexemplify the general characteristics of the methods, algorithms, anddevices among those of the present technology, for the purpose of thedescription of certain aspects. These figures may not precisely reflectthe characteristics of any given aspect and are not necessarily intendedto define or limit specific embodiments within the scope of thistechnology. Further, certain aspects may incorporate features from acombination of figures.

DETAILED DESCRIPTION

The present teachings provide an improved active catalyst for thetreatment of a low temperature exhaust gas stream. The catalyst promotesthe direct removal of nitrogen oxides from the exhaust gas stream. Thelow temperature, direct decomposition is accomplished without the needof a reductant (i.e., H₂, CO, C₃H₆, other hydrocarbons, and/or soot),thereby allowing improving fuel efficiency. Direct decomposition, asdiscussed herein, refers to catalytic transformation of nitrogen oxidesto elemental nitrogen and oxygen. The catalyst is an (amorphousCuO_(x))/Co₃O₄ catalyst where an amorphous layer is deposited byprecipitation of CuO_(x) from solution onto a Co₃O₄ support.

The presently disclosed catalyst system includes a method for dispersingcopper oxide on a metal oxide support, specifically a Co₃O₄ spinel oxidewith known N₂O decomposition activity. The deposition is carried out byforming a layer of CuO_(x) from an aqueous solution using a precipitantsolution under controlled pH conditions. Upon washing and calcinating,the (amorphous CuO_(x))/Co₃O₄ catalyst is found to be active withloadings displaying some amorphous CuO_(x) up to about 10 weightpercent, which exceeds that possible using an incipient wetnessimpregnation technique. Crystalline CuO in the amorphous CuO_(x) isobservable at about seven weight percent. The activity of theprecipitated (amorphous CuO_(x))/Co₃O₄ catalyst at about five weightpercent amorphous CuO_(x) loading (5.1 wt. %) is roughly fifteen percentgreater than is that from the optimized catalyst prepared by theincipient wetness impregnation technique (4.6 wt. %).

As detailed herein, the present teachings not only include thedevelopment of the catalyst system, but also the use of the catalystsystem with exhaust gas streams, particularly with catalytic convertersfor vehicles, automobiles, and the like, employing internal combustionengines.

The catalyst system of the present disclosure can be used in a chamberor an enclosure, such as a catalytic converter, having an inlet and anoutlet. As known to those of ordinary skill in the art, such a chamberor enclosure can be configured to receive an exhaust gas stream throughthe inlet and to discharge the exhaust gas stream through the outlet,such that the exhaust gas stream has a particular or defined flowdirection.

In an aspect of the invention, the (amorphous CuO_(x))/Co₃O₄ catalystdisplays an amorphous CuO_(x) layer over a range of about three to aboutten weight percent on the Co₃O₄ support. In contrast, catalysts formedby deposition of CuO_(x) by the incipient wetness impregnation techniquedisplay an amorphous CuO_(x) monolayer up to about 2.3 weight percent,where the catalyst displays optimal NO_(x) reduction activity. At higherCuO_(x) loadings by the incipient wetness impregnation technique, theCuO_(x) becomes crystalline and a decreased activity is observed.Deposition of the CuO_(x) by the precipitation method results in anamorphous structure to higher weight percentages on the (amorphousCuO_(x))/Co₃O₄ catalyst. Where the amorphous CuO_(x) deposition isformed at about five weight percent, the direct NO decompositionactivity is roughly fifteen percent greater than that from the optimalCuO_(x)/Co₃O₄ catalyst prepared by the incipient wetness impregnationtechnique, employing essentially equivalent Co₃O₄ spinel oxide support.

In an aspect of the invention, a method for preparing the (amorphousCuO_(x))/Co₃O₄ catalyst is carried out by employing a precipitationmethod. A suspension of particulate Co₃O₄ is prepared in a solution ofCu(NO₃)₂. The proportions of spinel oxide particles to Cu(NO₃)₂ iscalculated for a desired CuO_(x) loading on the support. A solution of abasic precipitation agent is slowly added to an agitated suspension. Theprecipitation agent can be Na₂CO₃, K₂CO₃, Li₂CO₃, Rb₂CO₃, Cs₂CO₃,Fr₂CO₃, or any combination thereof. The basic precipitation agent isadded until the pH of the suspension achieves a level of about 9 to 10.Subsequent isolation of the resulting precipitate coated particles andwashing with purified water results in effectively salt free suspendedsolids. After filtration and drying, the solids are ground into a finepowder and calcined at an elevated temperature.

The deposition-precipitation method, according to an aspect of theinvention, can be carried out using a batch or continuous process forcombination of the precipitant solution and the suspension. In thismanner an aqueous copper nitrate solution is prepared to which theaddition of the cobalt oxide particles results in a suspension. Theparticles can be provided with an average particle size of from about 5to 1,500 nm. The Co₃O₄ support particles can be nanoparticles, forexample, but not limited to, those with an average cross-section of fromabout 2 to about 100 nm, and in one non-limiting, example 21 nm. Theslow addition of the basic precipitant solution can be carried out usingat least one dropping funnel, or its equivalent, or at least one pump,where the profile of the addition is maintained to a desired rate andthe rate can be accelerating or deaccelerating, such that the qualityand throughput of the precipitated (amorphous CuO_(x))/Co₃O₄ catalystprecursor is optimized. The appropriate agitation can be provided by atleast one stirrer or other mixers. A continuous mixing loop can beconstructed employing at least one active or passive inline mixer with aflow of the suspension through the loop. Alternately or additionally,mixing can be performed or augmented by cavitation that can be promotedby ultrasonic, piezoelectric, or other means.

Subsequent to formation of the precipitated (amorphous CuO_(x))/Co₃O₄catalyst precursor, removal of the resulting aqueous solution from theparticles can be carried out by filtration or centrifugation. Afiltration can be performed by imposing a pressure on the particleproximal side of a filter or reducing the pressure on the particledistal side of the filter. Subsequently, the solid particles are washedto remove water soluble salts. The washing can be carried out in a batchmanner, where the particles are suspended in a purified water andre-filtered or re-centrifuged; or the particles are washed in acontinuous manner by flowing purified water through the filter bed orcentrifuge bed. The purified water can be distilled water, ion-exchangedwater, or reverse osmosis purified water.

The washed (amorphous CuO_(x))/Co₃O₄ catalyst precursor is dried. Dryingcan occur in an air, under nitrogen, an inert atmosphere, an oxygen richatmosphere, or under vacuum. Drying can occur at temperatures fromambient to 120° C. depending upon the pressure employed during drying.The dried (amorphous CuO_(x))/Co₃O₄ catalyst precursor is ground to afine or superfine powder. Grinding can be carried out in any millappropriate for the hardness of the material. For example, the mill canbe, but is not limited to, a ball mill, a high compression roll mill, aroll mill, or a universal mill. Subsequently, the dried (amorphousCuO_(x))/Co₃O₄ catalyst precursor is calcined at temperatures in excessof 300° C., for example, but not limited to, 450° C. but less than 500°C., to provide the (amorphous CuO_(x))/Co₃O₄ catalyst.

The (amorphous CuO_(x))/Co₃O₄ catalyst can be characterized by X-raypowder diffraction (XRD) and X-ray photoelectron spectrometry (XPS). Ascan be seen in FIG. 1, for a 5 wt. % loading of CuO_(x) from the variousprecipitating agents, an amorphous CuO_(x) results as indicated by alack of reflections from crystalline copper oxides. As shown in FIG. 2,as the loading of CuO_(x) increases, XRD analysis of the catalyst powderdisplays reflections due to the Co₃O₄ spinel oxide but no reflectionsdue to CuO_(x), CuO, or other cobalt oxides in three and five weightpercent samples, but displays some CuO at seven and 10 weight percentloading of CuO_(x). Analysis by XPS, as shown in FIG. 3 and FIG. 4,under otherwise equivalent conditions, yields spectra whose features canbe further analyzed to determine the relative quantity of CuO_(x)residing on the Co₃O₄ support is affected by the identity of theprecipitating agent. Clearly, use of the alkali metal carbonate, Na₂CO₃results in higher loadings than the alkali metal hydroxide, NaOH, or theammonium carbonate; which is given in Table 1 below where the Cu2p/Co2pratio is given for the three precipitating agents.

TABLE 1 Cu2p/Co2p atomic ratios for (amorphous CuO_(x))/Co₃O₄ catalystssynthesized using various precipitating agents Precipitating AgentCu2p/Co2p Ratio Na₂CO₃ 0.27 NaOH 0.24 (NH₄)₂CO₃ 0.05

The differences displayed in structure of the (amorphous CuO_(x))/Co₃O₄catalysts from different precipitating agents is reflected in the directNO decomposition activity, as can be seen in FIG. 5 for 5 wt. %loadings. The activity of the catalyst from Na₂CO₃ is clearly higherthan those from NaOH and (NH₄)₂CO₃ at all temperatures in excess of 350°C. The activity is at its maximum at about 450° C. except for catalystfrom the ammonium carbonate precipitating agent, which displays higheractivity at higher temperatures. The catalytic activity depends upon thedegree of loading of the CuO_(x) on the Co₃O₄ support, as is illustratedin FIG. 6 for the amorphous CuO_(x)/Co₃O₄ catalyst prepared using sodiumcarbonate as the precipitating agent. At all CuO_(x) loadings, theactivity is greater at 450° C. than 500° C. At both temperatures, theactivity is greatest at the 5 wt. % loading of the CuO_(x) relative tothe loadings of 3, 7, and 10 wt. %.

EXAMPLES

Various aspects of the present disclosure are further illustrated withrespect to the following Examples. It is to be understood that theseExamples are provided to illustrate specific embodiments of the presentdisclosure and should not be construed as limiting the scope of thepresent disclosure in or to any particular aspect.

Methods

XRD patterns are obtained using Rigaku SmartLab X-ray Diffractometerusing Cu Ka radiation (|¼ 1.5405 A) with a glass holder as the samplesupport. The scanning range is from 10 to 80 (2θ) with a step size of0.02 and a step time of 1 s. The XRD phase present in the samples isidentified in reference to ICDD-JCPDS data files.

XPS measurements are performed using PHI 5000 Versa Probe II X-rayPhotoelectron Spectrometer using an Al Kα source. Survey scans (with187.85 eV pass energy at a scan step pf 0.8 eV) and high resolution(O1s), (Co2p), (Cu2p), and (C1s) scans (with 23.5 eV pass energy at ascan step of 0.1 eV) were performed. Changing the catalyst samples wascorrected by setting the binding energy of the adventitious carbon (Cis)to 284.6 eV. The XPs analysis was performed at ambient temperature andat pressures typically on the order of 10⁻⁷ Torr. Prior to the analysis,the samples were outgassed under vacuum for 30 minutes.

Direct NO_(x) decomposition measurements were performed in a fixed bedflow reactor using ˜1% NO_(x) balance helium with a gas hourly spacevelocity of 2,100 h⁻¹ at a temperature region of 500° C. Catalystspretreatment was at 450° C. in the presence of 20% 02/He. Afterpretreatment, bed temperature was cooled to 350° C. in the presence ofHe. Direct NO_(x) decomposition measurements were collected in thetemperature range of 350° C. to 500° C. using 50° C. intervals.

Synthesis of (Amorphous CuO_(x))/Co₃O₄ Catalyst

(Amorphous CuO_(x))/Co₃O₄ catalysts having a five weight percent CuO_(x)deposition were synthesized where a basic precipitating agent, either(NH₄)₂CO₃, NaOH, or Na₂CO₃, was added to a Co₃O₄ spinel oxide suspensionin a Cu(NO₃)₂ solution. The solution of Cu(NO₃)₂ was formed by combiningthe salt with deionized water, to which Co₃O₄ spinel oxidenanoparticles, at 19 times the mass of the Cu(NO₃)₂ in solution, weremixed by stirring to form the suspension. To the stirred suspension wasadded a 0.25 to 0.75 M precipitating agent. Using a pH meter, theprecipitating agent was added dropwise until 200 mL of the solutionresulted in a pH of 9-10. The stirring was stopped, the supernatantsolution was decanted, and subsequently the remaining solution wasfiltered from the solids using filter paper. The filtered solids werewashed multiple times with distilled water. The washed solids were driedovernight in an oven at 120° C. Using a mortar and pestle A fine powderwas prepared. The fine powder was calcined in air at 2° C./min to 450°C. where it was maintained at that temperature for 1 hour.

The preceding description is merely illustrative in nature and is in noway intended to limit the disclosure, its application, or uses. As usedherein, the phrase at least one of A, B, and C should be construed tomean a logical (A or B or C), using a non-exclusive logical “or.” Itshould be understood that the various steps within a method may beexecuted in different order without altering the principles of thepresent disclosure. Disclosure of ranges includes disclosure of allranges and subdivided ranges within the entire range.

The headings (such as “Background” and “Summary”) and sub-headings usedherein are intended only for general organization of topics within thepresent disclosure, and are not intended to limit the disclosure of thetechnology or any aspect thereof. The recitation of multiple embodimentshaving stated features is not intended to exclude other embodimentshaving additional features, or other embodiments incorporating differentcombinations of the stated features.

As used herein, the terms “comprise” and “include” and their variantsare intended to be non-limiting, such that recitation of items insuccession or a list is not to the exclusion of other like items thatmay also be useful in the devices and methods of this technology.Similarly, the terms “can” and “may” and their variants are intended tobe non-limiting, such that recitation that an embodiment can or maycomprise certain elements or features does not exclude other embodimentsof the present technology that do not contain those elements orfeatures.

The broad teachings of the present disclosure can be implemented in avariety of forms. Therefore, while this disclosure includes particularexamples, the true scope of the disclosure should not be so limitedsince other modifications will become apparent to the skilledpractitioner upon a study of the specification and the following claims.Reference herein to one aspect, or various aspects means that aparticular feature, structure, or characteristic described in connectionwith an embodiment or particular system is included in at least oneembodiment or aspect. The appearances of the phrase “in one aspect” (orvariations thereof) are not necessarily referring to the same aspect orembodiment. It should be also understood that the various method stepsdiscussed herein do not have to be carried out in the same order asdepicted, and not each method step is required in each aspect orembodiment.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations should not beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A catalyst system for the direct decompositionremoval of NO_(x) from an exhaust gas stream provided at a temperatureof less than about 600° C., the catalyst system comprising an (amorphousCuO_(x))/Co₃O₄ catalyst comprising an amorphous CuO_(x) deposit onsurfaces of Co₃O₄ spinel oxide particles and configured to catalyzereduction of the NO_(x) to generate N₂ without the presence of areductant, wherein the deposit is equal to or greater than 3 weightpercent of the Co₃O₄ spinel oxide.
 2. The catalyst system according toclaim 1, wherein the amorphous CuO_(x) deposit comprises a precipitatedamorphous CuO_(x), wherein the precipitated amorphous CuO_(x) is formedfrom a mixture of Cu(NO₃)₂ and an alkali metal carbonate.
 3. Thecatalyst system according to claim 2, wherein the alkali metal carbonateis Na₂CO₃.
 4. The catalyst system according to claim 1, wherein thedeposit is about 5 weight percent of the Co₃O₄ spinel oxide.
 5. Acatalytic converter for the direct decomposition removal of NO_(x) froman exhaust gas stream flowing at a temperature of less than or equal toabout 600° C., the catalytic converter comprising: an inlet configuredto receive the exhaust gas stream into an enclosure; an outletconfigured to allow the exhaust gas stream to exit the enclosure; and acatalyst system contained inside the enclosure, the catalyst systemcomprising an (amorphous CuO_(x))/Co₃O₄ catalyst comprising an amorphousCuO_(x) deposit on surfaces of Co₃O₄ spinel oxide particles andconfigured to catalyze reduction of the NO_(x) to generate N₂ withoutthe presence of a reductant, wherein the deposit is equal to or greaterthan 3 weight percent of the Co₃O₄ spinel oxide.
 6. The catalyticconverter according to claim 5, wherein the Co₃O₄ spinel oxide particlesare nanoparticles, having an average diameter of from about 2 to about100 nm.
 7. The catalytic converter according to claim 5, wherein thedeposit comprising amorphous CuO_(x) is a precipitate of CuO_(x) ontothe Co₃O₄ spinel oxide particles, wherein the precipitate is formed froma mixture of Cu(NO₃)₂ and an alkali metal carbonate.
 8. The catalyticconverter according to claim 7, wherein the alkali metal carbonate isNa₂CO₃.
 9. The catalytic converter according to claim 5, wherein thedeposit is 5 weight percent of the Co₃O₄ spinel oxide.
 10. The catalyticconverter according to claim 5, configured to flow the exhaust gasstream through the catalyst system at a temperature at or less thanabout 500° C.
 11. The catalytic converter according to claim 5,configured to flow the exhaust gas stream through the catalyst system ata temperature at or less than about 400° C.
 12. A method for directdecomposition removal of NOx from an exhaust gas stream, the methodcomprising: flowing the exhaust gas stream through a catalyst system andcontacting the exhaust gas stream to a multiplicity of surfaces of thecatalyst system, the catalyst system comprising an (amorphousCuO_(x))/Co₃O₄ catalyst comprising an amorphous CuO_(x) deposit onsurfaces of Co₃O₄ spinel oxide particles, and configured to catalyze areduction of the NO_(x) to generate N₂ without the presence of areductant, wherein the deposition is equal to or greater than 3 weightpercent of the Co₃O₄ spinel oxide.
 13. The method according to claim 12,wherein the deposit comprises amorphous CuO_(x) as a precipitate ofCuO_(x) onto the Co₃O₄ spinel oxide particles, wherein the precipitateis formed from a mixture of Cu(NO₃)₂ and an alkali metal carbonate. 14.The method according to claim 10, comprising flowing the exhaust gasstream through the catalyst system at a temperature at or less thanabout 500° C.
 15. The method according to claim 10, comprising flowingthe exhaust gas stream through the catalyst system at a temperature ator less than about 400° C.
 16. A method for preparing an (amorphousCuO_(x))/Co₃O₄ catalyst for the direct decomposition removal of NO_(N),comprising: providing a suspension comprising Co₃O₄ spinel oxideparticles in a solution of Cu(NO₃)₂; precipitating a copper comprisingdeposit on the Co₃O₄ spinel oxide particles by the mixing of aprecipitant solution comprising an alkali metal carbonate with thesuspension to form an (amorphous CuO_(x))/Co₃O₄ catalyst precursorsuspension, washing the water-soluble components from the (amorphousCuO_(x))/Co₃O₄ catalyst precursor suspension with deionized water;isolating wet (amorphous CuO_(x))/Co₃O₄ catalyst precursor; drying thewet (amorphous CuO_(x))/Co₃O₄ catalyst precursor to form a dry(amorphous CuO_(x))/Co₃O₄ catalyst precursor; and calcining the dry(amorphous CuO_(x))/Co₃O₄ catalyst precursor to a temperature of 450° C.to form the (amorphous CuO_(x))/Co₃O₄ catalyst.
 17. The method accordingto claim 16, wherein mixing comprises adding the precipitant solution tothe suspension at a rate that permits maintenance of a pH below 10 andmixing continues until the pH is in the range of 9 to
 10. 18. The methodaccording to claim 16, wherein the alkali metal carbonate is Na₂CO₃. 19.The method according to claim 16, wherein calcining is by heating thedry (amorphous CuO_(x)/Co₃O₄ catalyst precursor at a rate of about 2° C.per minute to a temperature of about 450° C.